Blow - To leave, depart; e.g., "Did she blow last night?"
Brick - A term used to describe a pound or kilogram of any drug; in the case of this film, it's heroin.
Bulls - Cops; e.g., "What first, tip the bulls?" Also, as a verb, to turn over to the cops; e.g., "I bulled the rat."
Burg (or Burgh) - Town, City; e.g., "He knows every two-bit toker in the burg."
C - Roman numeral for 100; e.g., "He had a C-Note."
Clam - To keep your mouth shut; e.g., "The muscle blows or I'll clam."
Copped - Obtained; e.g., "She copped the junk."
Dose - To take drugs; e.g., "He dosed off the bad junk and it laid him out."
Duck Soup - Easy pickings.
Gat - Gun.
Gum - To mess things up; e.g., "Bulls would only gum it."
Heel - To walk away from (and show your heels to); e.g., "I'm not heeling you to hook you."
Hook - Raise interest or get involved; e.g. "I'm not heeling to hook you."
Hop/Jake/Junk - Drugs. Back when noir films were more popular, (1940s-50s); hop is often used to reference marijuana and "hophead" is someone who smokes a lot of pot.
Knives in my eyes - a splitting headache; e.g., "I've got knives in my eyes. I'm going home sick."
Lap Dog - A lap dog is defined as a dog that is small enough to be held in the arms or lay comfortably on a person's lap. The term is also sometimes used to describe a person who is very easily controlled, such as a yes man. In the case of Brick both definitions are cleverly combined. If you recall the scene where we first meet Kara, she has a freshman boys head in her lap, who springs to her command of "run and get my purse."
On the Nail - immediately; e.g., "He wants cash on the nail."
Pick - A ride in a car (as in "pick-up"); e.g., "Did she get a pick?"
Raise - To get in touch with; e.g., "You couldn't raise em?" This is also used cleverly when Brain asks Brendan, "You couldn't raise her?" referring to Emily whom he could neither contact nor raise, as in raise from the dead.
Reef Worm - A stoner (reefer).
Scape - A patsy to take the blame (scapegoat/ fall guy).
Scraped - Begged off of, cadged from; e.g., "Ask any dope rat where their junk sprang and they'll say they scraped it off [name]...."
Shamus - A private detective.
Shine - To wield (as with a weapon); e.g., "He shines a blade."
Showing Your Ace - Obviously a card reference. An ace is the highest playing card, meaning 'high-quality. When Brendan strips Kara and says "I'm showing your ace," he is saying that Kara's most valuable asset is her beauty, which she uses to manipulate people with (e.g. lap dogs, Dode, etc). Not to mention, ace sounds similar to ass, something else of hers he shows off.
Specks - Eyes or Eyeglasses (spectacles); e.g., "Keep your specks peeled."
Sprang - Originated; e.g., "His gat sprang from Tugger's gang."
Squawk - To give up or snitch on. Or more apt to this, a stool pigeon.
Take A Powder - To slip away; e.g., "Why'd you take a powder the other night?"
Yegg - A criminal.